As per Microsoft: "An Entity Control Block is a segment of code that is created for each message transfer agent (MTA) and connector that a given MTA communicates with. The ECBs are created during MTA startup. By default, the Exchange Server MTA is configured for 25 ECB connections to 25 other Exchange Server computers in a given site. This issue can occur if this server count is exceeded". See ME205533 to solve this problem.